An "over-the-belt" barcode scanner system is a system in which a camera or other imaging device is positioned over or adjacent a pathway, such as a conveyer belt, to scan items which pass through a field of vision of the imaging device. Such a system is primarily designed to read any bar code in a wide field of view. The width of the searching area for over-the-belt scanner systems is typically from 30 cm up to 150 cm.
If the imager is positioned over the belt, the width of the belt defines the number of sensing elements necessary to achieve the desired resolution. For example, if the scanner system needs to read a bar code symbology with a 0.125 mm. (0.005 in) "x" dimension in a 75 cm wide field of view, then the minimum resolution has to be at least one pixel per element or, in this case, one pixel per 0.125 mm. The total number of required sensing elements in this case will be 750/0.125=6000 pixels. As we can see from this example, the width of the searching area in many practical applications is extremely large. The situation is significantly aggravated for two dimensional bar code symbologies such as Data Matrix.
Prior to reading a symbology, it is necessary to locate the symbology. In some cases, the symbology is not aligned with a specific scanner location, so that it is necessary to locate the symbology within a large field of view. The process of locating the symbology for subsequent reading of the symbology is called "finding." Early finding techniques included attempting to scan symbologies until a predetermined beginning or ending sequence of leading edges was detected, followed by further sequences which match a predetermined pattern. This technique is particularly well adapted to data scans of bar codes in similar one-dimensional symbologies. In the case of two-dimensional symbologies, such a scan is inadequate.
Various techniques have been used to find two-dimensional symbologies. These techniques include scanning for characteristics that are added to the symbology to identify a position of the symbology. Typical characteristics of this type include starting bars, center targets and finder patterns. For example, to identify the location of Data Matrix symbology, an L-shaped finder pattern is used to locate the symbology in the field of view of a video imager. Any straight line within the field of view is a candidate for the "L" searching algorithm. This means that in order to find the symbology, we must trace every straight line in an attempt to find another straight line which is perpendicular to the first line. A goal of the present invention is to provide a method of filtering the image in real time in order to eliminate all features which are distinguishable from the symbology.
It would be advantageous if one could efficiently and quickly perform the "finding" operation. Since the code itself need not be read, it would be advantageous if finding were accomplished in a different and simpler manner from the process of reading the code.